The throttle return spring closes the throttle. You really don't want too stiff a spring, as it pulls against the throttle arm and eventually causes wear on the shaft.

The accelerator pump spring has nothing to do with the throttle plates returning to closed. Binding anywhere in the linkage does.

The first time I reassembled a QJet after many years without touching one, I got the plastic choke cam/weight on the wrong side of the lever. It did just as you describe. I pulled the choke assembly back off, raised the cam, and put it back together. Take a look at those parts to be sure nothing is binding.